It is often desirable to monitor particular characteristics of a certain component of interest found in a medium. For example, the monitoring of phosphate levels in sewage is useful to determine whether the sewage has been adequately treated. Similarly, in virtually any chemical processing system, knowledge of the concentration of a particular component in solution can be useful for system control. Further, in biomedical applications, information concerning the concentration of certain components in body fluids such as blood can be extremely important to the proper diagnosis of a patient's condition.
Given the diversity of characteristics, components, and media that may be involved, a system, including a measuring instrument and a sensor, that is able to accurately produce measurements under these different conditions is subject to numerous design constraints. For example, the system may be required to provide a selective response to a particular characteristic and component of the media. The system may also be called upon to automatically respond to changes in the characteristic of the component, providing updated information with minimal operator intervention. In certain applications, the system sensor preferably would have a relatively long life and be reuseable without adverse effect upon its accuracy. Alternatively, it may be desirable in some instances to employ a sensor that is disposable. Finally, it would be desirable to produce a system that is either precalibrated, or requires no calibration, and whose size, complexity and cost are kept to a minimum.